Well, it all depends on what you're taking and why, as to how long you need to take something for it to "work". My father was in the vitamin business for 30 years so I come by the info honestly :)
The general rule of thumb is that you need to take herbs or vitamins for 2 weeks before you build up adequate levels in your system. Multivitamins fall under this category, but if you are taking a prenatal supplement, it is recommended that you take them a minimum of 3 months prior to ttc, because it takes that long for the folic acid to provide the maximum benefit to the fetus.
Yes, I am taking a Prenatal vit because I am ttc, I started taking them a few months ago when i started ttc, but since i didn't get preg right away i stopped, but I am serious about wanting to concieve, so i started up again..thats why i ask..Thanks!